Munn semigroup
# Munn semigroup

In mathematics, the Munn semigroup is the inverse semigroup of isomorphisms between principal ideals of a semilattice (a commutative semigroup of idempotents). Munn semigroups are named for the Scottish mathematician Walter Douglas Munn (1929–2008).

## Construction's steps

Let $E$ be a semilattice.

1) For all e in E, we define Ee: = {i ∈ E : i ≤ e} which is a principal ideal of E.

2) For all e, f in E, we define $T_{e,f}$ as the set of isomorphisms of Ee onto Ef.

3) The Munn semigroup of the semilattice E is defined as: $T_E$ := $\bigcup _{e,f\in E}$ { $T_{e,f}$ : (e, f) ∈ U }.

The semigroup's operation is composition of partial mappings. In fact, we can observe that $T_E$ ⊆ $I_E$ where $I_E$ is the symmetric inverse semigroup because all isomorphisms are partial one-one maps from subsets of E onto subsets of E.

The idempotents of the Munn semigroup are the identity maps $1_{Ee}.$

## Theorem

For every semilattice $E$, the semilattice of idempotents of $T_{E}$ is isomorphic to E.

## Example

Let $E=\{0,1,2,...\}$. Then $E$ is a semilattice under the usual ordering of the natural numbers ($0<1<2<...$). The principal ideals of $E$ are then $En=\{0,1,2,...,n\}$ for all $n$. So, the principal ideals $Em$ and $En$ are isomorphic if and only if $m=n$.

Thus $T_{n,n}$ = {$1_{En}$} where $1_{En}$ is the identity map from En to itself, and $T_{m,n}=\emptyset$ if $m\not =n$. The semigroup product of $1_{Em}$ and $1_{En}$ is $1_{E\operatorname {min} \{m,n\}}$. In this example, $T_{E}=\{1_{E0},1_{E1},1_{E2},\ldots \}\cong E.$

